My first step to contributing

Let this be my first thread to understanding ISPConfig3 code structure so that I can contribute. My first contribution is add a new API function for downloading and restoring of backup files through API.

Please help as much as possible in this thread to achieve that goal. Then I can hit more complex functions.

- The permission check is missing.
- Errors are not returned as soap faults.
- The params array contains only one value if I see it correctly, you should use $action as parameter and not $params array.